Treatment of aluminate digester liquor

ABSTRACT

Alumina-containing minerals, such as bauxite, are digested in sodium aluminate liquor, wherein the digestion liquor leaves the digester at a temperature above 150*C and is conducted, without flashing, in a heat exchanger in a countercurrent flow with respect to the feed liquor for the digester so that the temperature difference between the two liquors is less than 10*C.

United States Patent 1 Lazar TREATMENT OF ALUMINATE DIGESTER LIQUORInventor:

Ferenc Lazar, Tatabanya, Hungary Assignee: Tatabanyai Szenbanyak,Tatabanya,

Hungary Filed: July 31, 1970 Appl. No.: 59,954

[30] Foreign Application Priority Data Aug. 5, 1969 Austria A 7521/69US. Cl. 159/47 R, 159/2 MS, 159/46,

Int. Cl 801d 7/00, BOld 9/00, F26b 3/12,

'B0ld 3/02, C01f7/34, BOld 11/04 Field of Search 159/2, 2 MS, 4 MS,

159/46, 17 VS, 45, 47, DIG. 8, DIG. 25; 202/173; 23/143, 297-299,305-306 56] References Cited UNITED STATES PATENTS 2,806,766 9/1957Anderson 23/143 BAUXITE- CAUSTIC SLURRY I HOLDING TANK r-usu vzsssrs 11]3,738,411 June 12, 1973 2,852,343 9/1958 Scandrett et al 23/1433,497,317 2/1970 Tusche 23/143 2,459,302 1/1949 Arouson 159/47 WL X2,824,058 2/1958 Zimmerman 159/47 WL X 3,332,470 6/1967 Chirico et a1.159/47 FOREIGN PATENTS OR APPLICATIONS 780,291 3/1968 Canada 23/143851,900 10/1952 Germany 159/47 WL Primary Examiner-Norman YudkoffAssistant Examiner-J. Sofer Attorney-Gabriel P. Katona [57] ABSTRACTAluminum-containing minerals, such as bauxite, are digested in sodiumaluminate liquor, wherein the digestion liquor leaves the digester at atemperature above 150C and is conducted, without flashing, in a heatexchanger in a countercurrent flow with respect to the feed liquor forthe digester so that the temperature difference between the two liquorsis less than 10C.

4 Claims, 1 Drawing Figure PRESSURE R EGULATOR l DIGESTER LIQUOR 4SLURRY V II 1 DILUTION TANK 2Q TREATMENT OF ALUMINATE DIGESTER LIQUORThe digestion of alumina containing minerals, e.g., of bauxite, iscarried out, as known, in a continuously operating digester line withsodium aluminate liquor at temperatures above 150C and under pressureaccording to the Bayer process. An increase of the concentration ofcaustic Na O (e.g., above 220 g/l caustic Na O) to be designatedhereinafter in the specification and the claims as kxNa O in thedigesting liquor is advantageous for the digestion, since higher causticconcentrations allow the application of lower temperatures and reducedpressure, further specific pressure may also be lowered compared to theprocess in which the digesting liquor has a lower concentration of kx NaO.

Nevertheless the recent trend is to perform bauxite digestion withdilute liquors (below 220 g/l kXNa O), since thereby the amount ofdilute liquor to be evaporated will be diminished resulting in a savingin steam and elimination of the maintenance costs which compensate forthe drawbacks of the digestion process.

If, however, digestion is carried out with a dilute liquor a smalleramount of wash water will be sufficient to eluate the caustic which hasbeen introduced into the cycle and this fact causes caustic losses.Compared to the saving in steam this loss causes, however, only arelatively insignificant rise in costs, provided multistep settlers areused and at the end of the line the filtration of the red mud is carriedout e.g., by means of filter presses. Increasing the concentration ofthe digesting liquor is achieved by means of evaporating the diluteliquor which has been obtained in the course of precipitation.

In alumina production water has to be evaporated not only whenconcentrating the dilute liquor obtained after precipitation of aluminumhydroxide, but evaporation results also due to the expansion of thedigester liquor leaving the digester, which is at a temperature above150C. Since the evaporation of the digester liquor takes place onlyafter digestion, its occurrence does not result in any particularadvantage in the bauxite decomposition process. This is because duringthe flashing the temperature of the resulting concentrated slurrybecomes so low that it has no significance in the digestion. Digestionwith dilute liquor involves as a rule the evaporation ofless water-fromthe spent liquor than the quantity of water which evaporates when the digester liquor is subjected to expansion. This fact is illustrated on anexample in the following table.

liquor liquor liquor Digesting after after after liquor mixing expansionof precipitation prior to the digester of the alumidigestion liquor nateliquor kxNa,O 180 g/l 172 236 145 m of water per metric ton of alumina12 12.57 9.17 14.90

In case of a digester liquor with 180 g/l of kxNa O concentration thetotal quantity of water leaving the cycle will therefore be 6.30 m permetric ton of alumina of which 2.90 m of water per metric ton of aluminacan be removed by evaporation in an evaporator and 3.40 m of water permetric ton of alumina by means of the expansion of the spent liquor.

Evaporation of the dilute liquor is absolutely necessary, for adigesting liquor with a concentration lower than 170 g/l kxNa O isuneconomical due to a deterioration in digestion efficiency. Anothermethod of eliminating the evaporation of the liquor may be carried outby diluting the digester liquor only to a degree which will ensure thatafter decomposition and precipitation of the aluminum hydroxyde, aconcentration of 180 g/l of kxNa O will obtain. This solution is howeveruneconomical partly because of the less satisfactory settling of the redmud and partly because of a lowering of the decomposition efficiency.

By applying the method and apparatus in accordance with the presentinvention evaporation of the dilute liquor can be eliminated withoutcausing a reduction in the efficiency of the process.

The method for the processing of digester liquor slurry obtained in thecontinuous digestion of alumina containing minerals, especially ofbauxite, with sodium aluminate liquor according to the present inventionis characterized by leading the liquor leaving the digester at atemperature above 150C without expansion in indirect countercurrent heatexchange with respect to the dilute liquor used for digestion throughheat exchangers or through series connected heat exchangers preferablyso that the temperature difference between the dilute liquor exitingfrom the heat exchanger and of the feed digester liquor slurry enteringthe heat exchanger should be less than 10C and evaporating the heateddilute liquor in one or several steps by expansion (flashing).

The sodium aluminate liquor used for the digestion of bauxite is aliquor which is obtained by the dilution of the digester liquor andseparation of the red mud by decomposition.

The digester liquor is diluted to a value suitably of about to g/l ofkxNa O.

The dilute liquor to be evaporated and used for digestion is preferablyled at pressure values of 5 to 60 atm. through the heat exchanger,thereafter the flashing of the heated dilute liquor leaving the heatexchangers is effected in several steps.

The super-heated steam formed by the flashing of the dilute liquor isused for pre-heating the starting mixture.

The apparatus for the implementation of the method according to thepresent invention is characterized by the used of at least one heatexchanger between the digester and the flashing (expansion) equipmentsaid heat exchanger being fitted on one end with a conveyor pump to feedthe dilute liquor to be evaporated and at the opposite end of the heatexchanger with a pipe for feeding the digester liquor, and withautomatic temperature and pressure control devices.

As heat exchanger preferably a shell tube heat exchanger is used whichwithstands operation pressures up to 100 atm., or heat exchangers withlaminar plates or of helical construction withstanding operationpressures pressures between 15 and 50 atm.

For most effective heat exchange it is advantageous to have on the onehand a desilicated digester liquor and on the other hand solids of smallparticle sizes in the digester liquor. The diameter of the particles isbetween of about 1 to about 45 microns.

According to one embodiment of the present invention in cases in whichpart of the liquor recirculated into the digestion process for theremoval of the inherent salts must be adjusted to a higher Na Oconcentration than the concentration of the digesting liquor, it ispossible to proceed in such a way, that a part of the flashed liquor isfed back into the container fitted before the heat exchanger Where saidliquor is reheated, i.e., evaporated, and part of the dilute liquor isled not through the heat exchanger, but mixed directly with theconcentrated liquor obtained after desalting and used as digestingliquor.

The dilute liquor to be evaporated is transported by means of piston orcentrifugal pumps capable of producing over-pressures of 5 to 60 atm.into the heat exchanger, while at the opposite side of the heatexchanger the pressure of the digester liquor may be used fortransporting it.

An embodiment of the equipment for the application of the method inaccordance with the present invention is shown on the attached FIGURE.

The digester liquor slurry leaving the digesters 17 and 18 at atemperature above 150C and under pressure is led through one of thechambers la, 2a and 3a of the series connected heat exchangers 1, 2 and3, while in the other chambers 11), 2b and 3b of the series connectedheat exchangers dilute liquor flows in countercurrent. The dilute liquororiginates from tank 6 which is designed to store the decomposed diluteliquor and is transported by means of pump 5 into the heat exchangerseries. In the series connected heat exchangers 1, 2 and 3 part of theheat content of the digester liquor slurry is transmitted to the diluteliquor preferably in such a way that the temperature difference betweenthe dilute liquor leaving from the series connected heat exchangers andthe introduced digester liquor slurry should be less than 10C. In theheat transfer process the desired temperature difference can be adjustedby means of a temperature regulator 7, and a pneumatic valve 8.Automatically controlled pressure regulators 19 and 20 serve to ensurethe heat transfer. The dilute liquor which leaves the heat exchangers ata temperature higher than its boiling point and under pressure is ledthrough flash vessels 9, 10 and 11 in which it is stepwise expanded andfinally through pipe 12 into a holding tank, for the concentrated liquorwhich is kept under atmospheric pressure for use in the treatment ofbauxite. The caustic is mixed with the bauxite and the resulting slurry13 is introduced through preheaters 14-16 into the digesters.

The digester liquor slurrywhich leaves the part 3a of the heat exchanger3 and has been cooled to below its boiling point is led through pipe 4into a dilution tank (not shown) which is also under atmosphericpressure. The red mud is separated from the slurry, the aluminate isprecipitated and separated by filtration and the canstic liquid isintroduced into the tank 6. The overheated vapors which leave the flashvessels 9, l0 and 11 are led as heating steam into preheaters 16, 15 and14, respectively, of the initial liquor. The starting bauxite slurryarriving through pipe 13 is heated in these preheaters.

The following examples illustrate the process of the present invention.Example 2 is intended as a comparison with the known process.

Example 1 3.1 metric ton of moist bauxite per metric ton of alumina ismixed with 12 or digesting liquor per metric ton of alumina, saiddigesting liquor having a concentration of 180 g/l of kxNa O. in thisway the quantity of the liquor increases after mixing to 12.57 m permetric ton of alumina, while its concentration drops to 172 g/l of kxNaO. This starting mixture is treated in a digester at a temperature of240C and under pressure of 40 to 60 atm. According to the invention thedigester liquor obtained in this way is led into a surface heatexchanger in which the liquor is cooled without flashing in. such a waythat in the heat exchanger the dilute liquor obtained afterdecomposition is led in countercurrent to the digester liquor. From thesurface heat exchanger the cooled digester liquor is led into thedilution tank, while the dilute liquor flowing in countercurrent is ledin 10 steps through a flash equipment in which it expands to atmosphericpressure and the flashed steam of the dilute liquor is used for heatingthe starting mixture. Flashing of the dilute liquor is performed in sucha way that 3.4 m of water per metric ton of alumina is removed.

In the dilution tank the digester liquor is diluted to g/l of kxNa Oconcentration by means of the red mud liquor. The quantity of wash waterused for dilution is 3.4 m per metric ton of alumina. The smallerquantity of wash water which can be used for the washing of the red mudmay raise in this case the loss of caustic which can be washed out fromthe red mud, therefore at the end of the settling-washing line the mudis filtered to compensate for the caustic losses. The concentration ofthe spent liquor obtained after decomposition increases as a result ofthe removal of the hydrate crystal water to g/l of kxNa O, whereby inthe dilute liquor flash equipment of the digester plant by means offlashing of 3.4 m of dilute liquor per metric ton of alumina a digesterliquor with a kxNa O concentration of 180 g/] is obtained.

Hence the method according to the present invention eliminates theapplication of a separate liquor evaporator unit.

Example 2 3.1 metric ton of bauxite per metric ton of alumina isdigested with 12 m digesting liquor of a kxNa O con centration of 180g/l. After mixing of the bauxite with the digesting liquor the quantityof the liquor is increased by the moisture content of the bauxite to12.57 m per metric ton of alumina, while at the same time theconcentration of the liquor drops to 172 g/l of kxNa O. The startingmixture is heated in the digester to 240C under a pressure of 40 to 60atm. Under these conditions the alumina content of the bauxite dissolvesin the liquor. The digester liquor under pressure is led through a flashline having 10 stages where it is ex panded to atmospheric pressure andthe flashed steam is used for heating the liquor prepared for digestion.The digester liquor which has been cooled in this way to 107C is ledthen into the dilution tank. The digester liquor is concentrated in thisway to 236 g/l of kxNa O. The degree of water removal amounts to 3.4 mper metric ton of alumina, the volume is reduced to 9.17 m per metricton of alumina. 1n the course of dilution the digester liquor is dilutedwith red mud water to 140 g/l of kxNa O, but is concentrated duringdecomposition to g/l of kxNa O. The water requirement of dilutionamounts to 6.3 m per ton of alumina. From the dilute liquor obtainedafter decomposition a quantity of water of 2.90 m per metric ton ofalumina is evaporated in the liquor evaporator whereby a digestionliquor of a concentration of 180 g/l of kJcNa O is obtained. The waterremoval of 6.3 2.9 3.4 in per metric ton of alumina required for thewater balance of the cycle is ensured by the flashing of the digesterliquor.

What we claim is:

1. In a process for the treatment of aluminacontaining minerals, whereinthe aluminum content of the mineral is extracted in a reaction zone witha caustic solvent under elevated temperature to form a hot liquidslurry, the improvement which comprises transferring heat withoutexpansion in an indirect heat exchanger from the hot liquid slurry afterit leaves the reaction zone in countercurrent to a dilute causticliquor, and concentrating the thus heated dilute caustic liquor byflashing, wherein said dilute liquor is obtained by separating thesolids content and precipitating at least a major portion of thedissolved aluminum content of said liquid slurry after it leaves theheat exchanger.

2. The improvement of the process of claim 1, wherein the temperaturedifference between the hot liquid slurry entering the heat exchanger andthe dilute caustic liquor leaving the heat exchanger, is not greaterthan about 10C.

3. The improvement of the process of claim 1, wherein the dilute causticliquor which enters the heat exchanger contains from about to about g/lkx Na O and the pressure of the dilute caustic liquor in the heatexchanger is from about 5 atm. to about 60 atm.

4. The improvement of the process of claim 1 wherein the dilute causticliquor which was concentrated by flashing is slurried with analumina-containing mineral and the slurry is preheated prior to thedigestion-extraction thereof, the preheating being carried out byutilizing the steam liberated during flashing.

2. The improvement of the process of claim 1, wherein the temperaturedifference between the hot liquid slurry entering the heat exchanger andthe dilute caustic liquor leaving the heat exchanger, is not greaterthan about 10*C.
 3. The improvement of the process of claim 1, whereinthe dilute caustic liquor which enters the heat exchanger contains fromabout 120 to about 150 g/l kx Na2O and the pressure of the dilutecaustic liquor in the heat exchanger is from about 5 atm. to about 60atm.
 4. The improvement of the process of claim 1, wherein the dilutecaustic liquor which was concentrated by flashing is slurried with analumina-containing mineral and the slurry is preheated prior to thedigestion-extraction thereof, the preheating being carried out byutilizing the steam liberated during flashing.